What are TWIP High Entropy Alloys?

Preconditon for the design of twinning-induced plasticity high entropy alloys: drop the strict equimolar mixing rule!

Theoretical calculations have revealed that the strict rule of maximizing the configurational entropy is not a sufficent criterion to form massive solid
solutions. This opened the pathway towards designing non-equiatomic high entropy alloys. The plain
reason for this is that the shape of the entropy curve as a function of chemical composition assumes for most transition metal mixtures a rather flat course so that even larger compositional deviations from the originally
strict equiatomic alloying rule yield the quite similar entropy values as an equiatomic solid solution mix.

This thermodynamic relaxation of the originally quite strict high entropy alloy rule opens up a huge and very flexible alloying window for realizing true single phase states and it
allows for corresponding property tuning via cimpositional modification for example of the stacking fault energy.

Twinning-induced plasticity high entropy alloys

With this quite substantial additional compositional degree of freedom a hure realm of corresponding alloy variations can be explored to optimize the stacking fault energy and to transfer the
excellent nanotwinning induced cryogenic mechanical behavior observed also to room temperature. We have demonstrated this approach via the design of a lean (i.e. in terms of Co, Cr and
especially Ni content with respect to the equiatomic five-component alloy) twinning-induced plasticity high entropy alloy (TWIP-HEA), Fe40Mn40Co10Cr10 with excellent room
temperature mechanical properties that are comparable to those of advanced TWIP steels.

Interstitial TWIP high-entropy alloy

After our recent development of interstitial high-entropy alloys (iHEAs) we have studied and optimized these matierlals further to achieve alloys with an enhanced combination of strength and
ductility. In the current interstitial high entropy alloy these properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic
(FCC) gamma matrix into the hexagonal close packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these
mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was
produced and investigated in our group via in-situ and interrupted in-situ tensile testing
in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed
by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and
incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC gamma
matrix. Gamma regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC gamma grains with a size of ~10 mm can be activated at room
temperature at a stress below ~736 MPa.